Electrostatic discharge (ESD) events may occur as a result of a discharge of static electricity or electrostatic induction, typically from human or machine contact. Electronic devices and circuits can be harmed by the sudden current and voltage spikes caused by ESD, and ESD protection circuits have been developed to protect circuits against this potential damage.
Many standard ESD protection circuits use reverse-biased diodes connected in series. For example, two diodes may have common terminals connected to the input to a circuit and their other terminals connected to the supply rails. In normal operation the diodes are reverse-biased and act as an open circuit. When an ESD strike occurs with a high voltage at the input, one diode becomes forward biased and the ESD current can be directed to a connected core-clamp that limits the voltage of the strike and directs the current to ground, thus protecting the connected circuitry from the strike.
However, the non-ideal diodes contribute leakage current in the reverse-biased configuration. This leakage current can become large in many implementations and is undesirable, especially for devices using signals having a limited current range and/or having a limited supply of power. Minimizing such leakage current can become a major concern in the design of ESD protection circuits. The leakage current at the input can be the difference between the leakage currents of the two diodes, and these diodes may exhibit different leakage currents despite being of the same type, especially at higher temperatures. Furthermore, the leakage current may be proportional to the voltages across the diodes, such that leakage current can be a function of the input common-mode voltage. In addition, noise current may be of even more concern in some applications and is determined by the leakage current flowing through the diodes. Such current noise is additive, such that each diode contributes additional noise current to the input signal even in protection circuits that have biased the diode voltages to follow the input voltage.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.